Lithium-ion secondary batteries have high charged and discharged capacities, and are batteries being able to make the outputs high. Currently, the lithium-ion secondary batteries have been used mainly as power sources for portable electronic appliances, and have further been expected as power sources for electric automobiles anticipated becoming wide spread from now on. A lithium-ion secondary battery comprises an active material, which is capable of inserting and eliminating (or sorbing and desorbing) lithium (Li), in the positive electrode and negative electrode, respectively. Moreover, the lithium-ion secondary battery is operated by moving the lithium ions between both the electrodes within an electrolyte.
In order to enhance the safety of a lithium-ion secondary battery, intending to prevent internal short-circuiting by providing the surface of an active material layer with a protective layer has been investigated. For example, Patent Application Publication No. 1 (i.e., Patent Publication (KOKAI) Gazette No. 2008-159385) discloses that the rise of battery temperature at the time of overcharging is inhibited by a ceramic coating layer on an electrode surface. Patent Application Publication No. 1 further discloses the following: using as a binder in the ceramic coating layer a thermoplastic polymer of which the melting point is from 110° C. to 150° C.; coating from 30% to 90% of a ceramic surface with the binder; melting the thermoplastic polymer in the ceramic coating layer under a high temperature of 110° C. or more to coat a surface of the active material layer, and then inhibiting the contact between an active material and an electrolyte.
Moreover, Patent Application Publication No. 2 (i.e., International Publication No. 2010/050507) discloses to arrange a porous layer comprising an electric insulating material on the surface of a negative electrode active material layer in order to inhibit a battery from swelling at the time of charging and discharging. Patent Application Publication No. 2 also discloses that a thickness of the porous layer becomes smaller by employing particles with uniform particle diameter as an electric insulating material.
Basically, the thicker a protective layer becomes, the more the internal resistance of a battery augments, so that the charged and discharged capacities of the battery declines. Moreover, in order to maintain the energy density of a battery, a thinner protective layer is preferable. However, in order to secure safety of a battery, a protective layer needs to have a certain extent of thickness.
Moreover, when a protective layer is provided on an active material layer and a coarseness in the surface of the active material layer is large, the thickness of the protective layer is preferably the coarseness in the surface of the active material layer or more. Consequently, the thickness of the protective layer may lower charged and discharged capacities of a battery extremely.
From the above, an electrode for nonaqueous electrolyte secondary battery on which a protective layer enhancing safety without greatly lowering charged and discharged capacities has been desired.